If you look up at the night sky, how many stars can you see? If it’s a clear night, the answer is likely too many to count. There are, however, many different ways to estimate the number of stars that we can see with our naked eyes. Rough estimates reason that there are around 4500 stars, which is a lot. Now – for every star that you can see, there are approximately 2. x 1020 (222,222,222,222,222,222,222) stars that you can’t see. When considering that every star has at least one planet orbiting it, you end up with a ridiculously large number of planets. Despite this, NASA states that Earth is the “only one planet we know of so far that is teeming with life”. So, why doesn’t every planet harbor life? What makes Earth so special?
Throughout this reading, keep in mind the some of the main things that make a planet habitable:
A Blueprint for Life
Because Earth is, to our knowledge, the only planet that harbors life, the Earth is our “Only blueprint for life”. Therefore, all of our assumptions about habitable planets are based off Earth’s characteristics. For example, water plays a “crucial part in our own existence”, so naturally, we are going to take an interest in planets that contain liquid water.
The Goldilocks Zone
Although there are many different factors to consider when talking about what makes Earth, or any planet, for that matter, habitable, the first to usually come up is something called the Goldilocks zone. In essence, the goldilocks zone is the distance that a planet must be from its star in order to support liquid water. If a planet is too close to its sun, it will be too hot – the water will evaporate. If it’s too far, it will be too cold, and the planet will freeze. Earth, as it just so happens, lies in that goldilocks zone where water can exist in its liquid state. A planet’s location in this habitable zone plays a big role in what the planet will look like – whether that is a dry, desert-like planet, or a humid ice planet.
Our atmosphere is also responsible for life on Earth as we know it. First of all, our atmosphere traps and contains gases that complex life forms need to survive, such as oxygen and nitrogen. Our atmosphere also protects us from the majority of the sun’s harmful solar radiation. Without our atmosphere, the radiation from the sun would’ve killed off life a long time ago. The atmosphere is also pressurized, and without this pressure, liquid water wouldn’t be able to exist on the Earth’s surface. Furthermore, the gases in our atmosphere insulate our planet by trapping some of the sun’s energy. If you’re curious, here’s a list of things that would happen if the Earth’s atmosphere suddenly disappeared. Don’t worry, though, this is just a simple thought experiment.
While many planets have some kind of atmosphere, there are few that actually provide habitable conditions. For example, Mercury’s atmosphere is so thin that it might as well be a vacuum, and Venus’s atmosphere is very acidic and thick.
Star and Orbit
The relationship between a planet and its star is also important factor in determining whether or not that planet is habitable. The star has to be able to provide a constant, and stable source of energy, so that plants can photosynthesize. At the same time, the star has to have an appropriate spectral class: “early F or G, to mid-K”. These classifications indicate important characteristics of a star, such as its: temperature, luminosity, amount of ultraviolet radiation, radiation wavelength, and lifespan.
Another feature that researchers look at when searching for habitable planets is the orbit of the planet around it’s sun. The orbit must be stable yet not elliptical, or else the condition changes over the course of a year could prove too severe. The Earth also rotates on an axis, which allows for relatively consistent amounts of sunlight to warm its surface. This prevents one side of the planet rom being constantly hot and bright, while the other half is a frozen brick.
Adding to the already extremely long list of things that contribute to making a planet habitable, you need to consider what’s happening in its interior. Earth has a molten core, which is responsible for producing our Earth’s magnetosphere. Our magnetosphere protects us from the sun’s death rays (solar material), that otherwise would spell disaster for any life on Earth. Because of this, Earth’s magnetic field played a vital part in making Earth a habitable planet.
Who knows? Maybe one day, scientists will make a groundbreaking discovery that there are, in fact, aliens out there. Whether that’s a good or bad thing is a completely different question, that I will discuss in my next blog.